It is known to provide a hybrid electric vehicle having a powertrain that includes an engine and an electric propulsion motor. In so-called parallel hybrid vehicles the engine and electric propulsion motor are each operable to deliver tractive or motive torque to drive the vehicle.
In some parallel hybrid vehicles the powertrain is operable in a parallel mode in which the engine and propulsion motor deliver drive torque or an electric vehicle (EV) mode in which the electric propulsion motor alone delivers drive torque and the engine remains off.
In some known hybrid vehicles, if the vehicle is driving in EV mode and the propulsion motor is unable to deliver the amount of torque demanded by the driver, the vehicle may switch on the engine automatically in order to meet the driver demand for torque.
The present applicant has recognised that an amount of noise, vibration and harshness (NVH) associated with a transition of the powertrain from EV mode to parallel mode when driver demand is sufficiently high can be undesirable. Drivability of a vehicle can be adversely affected due to the time required for the transition to be made (i.e. the powertrain response time). Furthermore excessive component wear such as engine and/or clutch wear may be incurred by relatively sudden demands for torque from a stopped engine. Embodiments of the invention seek to mitigate one or more of the above mentioned problems.